Receivers in global navigation satellite systems (GNSS), such as the Global Positioning System (GPS), use range measurements that are based on line-of-sight signals from satellites. The receiver measures the time-of-arrival of one or more broadcast signals. This time-of-arrival measurement includes a time measurement based upon a coarse acquisition coded portion of a signal, called pseudo-range, and a phase measurement based on an L-band carrier signal, including L1 at 1.57542 GHz, L2 at 1.22760 GHz and will soon include L5 at 1.17645 GHz. Ideally, these measurements are based only on the direct line-of-sight signals. The actual signals received by the receiver, however, are a composite of the direct line-of-sight signals and one or more secondary reflected signals. These secondary signals, known as multi-path signals, are reflected by any number of structures, including buildings, equipment and the ground.
FIG. 1 illustrates a composite signal in a global navigation satellite system (GNSS) 100. A device 110 receives a direct-path signal 114 and a single multi-path signal 116 reflected off of object 112. The path-length of the multi-path signal 116 is longer than that of the direct-path signal 114. As a consequence, the multi-path signal 116 is a slightly delayed replica of the direct-path signal 114 with typically a lower amplitude. FIG. 2 illustrates a phasor diagram 200 of the signals received by the device 110 (FIG. 1) including in-phase I 1212 and quadrature Q 210 components (relative to an internal reference in the device 110 in FIG. 1). The quadrature Q 210 component has a 90° phase relationship with the in-phase I 1212 component. The direct-path signal 114 (FIG. 1) has amplitude Ad 214 and a phase θd 218. The multi-path signal 116 (FIG. 1) has amplitude Am 216 and phase θm 220. Since the multi-path signal 116 (FIG. 1) arrives at a different time than the direct-path signal 114 (FIG. 1), phases θd 218 and θm 220 are different.
Multi-path signals, such as the multi-path signal 116 (FIG. 1), give rise to a distortion in the L-band carrier signal also known as phase multi-path. FIG. 3 illustrates magnitude 310 as a function of time 312 for signals in phase multi-path distortion 300. A composite signal 314 received by the device 110 (FIG. 1) is the sum of a sinusoidal direct-path signal 316 and typically a lower-amplitude, delayed multi-path signal 318. The direct-path signal 316 and the multi-path signal 318 are encoded such that each undergoes a 180° phase reversal at a code chip edge. Note that the phase reversal is also known as a code transition. The code transition rate (also known as the code chip edge rate) is a sub-multiple of the L-Band Carrier Frequency. For example, in GPS the sub-multiple is 154 and 120 for the P-code on L1 and L2, respectively. The code chip rate is 1.023 MHz for the coarse acquisition code (or CA code). In many global navigation satellite systems (GNSS) signals are encoded with code transitions using a bi-phase modulation code where a carrier signal phase is advanced or retarded by 90°. The different phase of the multi-path signal 318 results in noticeable distortion in the composite signal 314 during time interval 320. There are, however, effects at other times, too. For example, in this illustration a zero crossing 324 of the composite signal 314 is delayed, i.e., shifted to the right, relative to a zero crossing 322 of the direct-path signal 316. In general, multi-path signals may result in zero crossings of the composite signal 314 that are delayed or advanced. This apparent phase advance or delay gives rise to a phase error.
There is a need for a technique to mitigate such a multi-path-induced error in global navigation satellite systems (GNSS).